We investigated expression of macrophage galactose/N-acetylgalactosamine-specific C-type lectin (MMGL) in normal mouse lung tissue and in lungs with metastatic nodules produced by OV2944-HM-1 mouse metastatic ovarian tumour cells. Cells expressing MMGL were detected in tissue sections using a rat monoclonal antibody (mAB) specific for MMGL, mAb LOM-14. The regions containing cells immunostained using mAb LOM-14 were restricted to the connective tissue surrounding blood vessels and respiratory epithelia, whereas alveolar regions of lung parenchyma were essentially devoid of these cells. In contrast, a significant number of cells in the alveolar regions was shown to express Mac-1 antigen (CD11b/CD18) and leucocyte common antigen (CD45). Immunoelectron microscopic study revealed the presence of MMGL in the intracellular vesicles of cells residing in connective tissue. In the tumour-bearing mice, MMGL-positive cells were also present within metastatic nodules. Their localization outside of the nodules was restricted to connective tissue. Cells with Mac-1 antigens were seen both in the nodules and in the alveolar regions. These results indicate that MMGL serves as a unique macrophage marker in mouse lung tissue due to its topographical site-dependent pattern of expression. The present results also suggest a possible involvement of macrophages expressing MMGL in the immune response directed against metastatic tumour cells.